This invention relates to apparatus for supplying alternating current to a load from a direct current source, and more specifically to a power supply capable of supplying alternating current to a load during intervals when coupling between the direct current source and the power supply is interrupted.
Modern day mass transit rail cars are constructed to include such conveniences as fluorescent lighting and air conditioning in an effort to improve passenger comfort and thereby encourage transit system ridership. Invariably, such transit car auxiliary loads require alternating current for operation. Since mass transit rail cars are usually energized from a direct current source coupled to the track rails, a separate AC power supply is required to convert direct current to alternating current to energize transit car auxiliary loads.
Heretofore, inverters, generally comprised of a plurality of pairs of solid state switching devices, with the switching devices of each pair coupled in series aiding fashion, and each of the pairs of switching devices coupled across the DC source, have been employed to convert direct current to alternating current. Because transit car auxiliary loads such as fluorescent lighting and air conditioning systems tend to draw considerably more current at start-up than during normal steady-state conditions, inverters which supply such transit car auxiliary loads must be sized to supply the peak magnitude of load current rather than the steady-state load current magnitude, thereby increasing the inverter cost. Use of a conventional inverter to supply transit car auxiliary loads with alternating current incurs the disadvantage that when a rail gap is encountered, interrupting the direct current supply to the inverter, transit car auxiliary loads are de-energized. When such loads are re-energized upon passage of rail gap, it may be necessary, if the interruption is of a lengthy duration, e.g 10-15 seconds, to re-synchronize auxiliary loads to the inverter.
The present invention concerns an AC auxiliary power supply for use on transit cars or the like which stores kinetic energy during intervals when auxiliary load current does not exceed its normal steady-state magnitude, and which converts kinetic energy to electrical current to supply all load current in excess of its normal steady-state magnitude and to supply the load with current during intervals of DC source interruption.